1. Field of the Invention
The invention relates to an air electrode for a metal-air battery capable of improving discharge capacity by being incorporated in a metal-air battery, and to a metal-air battery provided with this air electrode.
2. Description of Related Art
Metal-air batteries are rechargeable batteries that use an elemental metal or metal compound for the negative electrode active material and oxygen for the positive electrode active material. Since oxygen to be utilized as the positive electrode active material is obtained from the air, the necessity to seal the positive electrode active material in such batteries does not arise. This allows theoretically the metal-air batteries to be provided with greater capacity than secondary batteries that use a solid positive electrode active material.
In a lithium-air battery, which is a type of metal-air battery, the reaction of the following formula (I) proceeds at the negative electrode during discharge.2Li→2Li++2e−  (I)Electrons, generated in formula (I) and output to an external circuit, perform work in an external load, after which they reach the air electrode. Lithium ions (Li+) generated in formula (I) migrate within an electrolyte retained between the negative electrode and the air electrode from the negative electrode side to the air electrode side by electroosmosis.
In addition, the reactions of the following formulas (II) and (III) proceed at the air electrode during discharge.2Li++O2+2e−→Li2O2  (II)2Li+½O2+2e−→Li2O  (III)The lithium peroxide (Li2O2) and lithium oxide (LiO2) generated accumulate on the air electrode as a solid. During charging, the reverse reaction of formula (I) proceeds at the negative electrode, while the reverse reactions of formulas (II) and (III) proceed at the air electrode, thereby causing metal lithium to be regenerated at the negative electrode and enabling recharging.
Considerable research has been conducted in recent years on metal-air batteries using alkaline metal elements, including lithium, for the negative electrode active material. Japanese Patent Application Publication No. 2010-108904 (JP-A-2010-108904) discloses a metal-air battery having a negative electrode, which has a negative electrode layer containing a negative electrode active material having an alkaline metal element, and a negative electrode current collector that carries out current collection of the negative electrode layer, an air electrode, which has an air electrode layer containing an electrically conductive material, and an air electrode current collector that carries out current collection of the air electrode layer, and an electrolyte that conducts metal ions between the negative electrode and the air electrode, wherein the air electrode current collector is composed of a carbon material or a highly electron-conducting ceramic material.
An experimental example (Example 1) is described in JP-A 2010-108904 in which manganese dioxide (MnO2) is used for the air electrode. However, as indicated in the subsequently described examples, as a result of studies conducted by the inventors of the application, MnO2 was determined to have a small number of active sites on a catalyst.